This invention relates to the purification of a crude terephthalic acid product and, more particularly, to a process for substantially reducing the 4-carboxybenzaldehyde content of such product.
The growing commercial importance of terephthalic acid is significant. Polyesterification of this acid yields polyethylene terephthalate which is utilized in the production of fibers, tapes and injection-blow molded articles.
The economic production of terephthalic acid by the partial oxidation of p-xylene has been demonstrated. While this route to terephthalic acid is widely used, it produces a crude product which contains a number of impurities, most of which are incomplete oxidation products and which are more volatile than the terephthalic acid product. Of these impurities, 4-carboxybenzaldehyde is the most troublesome as it acts as a chain terminator during the polyesterification of the acid and, either alone or in combination with other intermediate oxidation products, has been found to give rise to undesirable colors in the resultant polyester product.
The oxidation of p-xylene generally yields a crude terephthalic acid product containing 0.1 to 3% 4-carboxybenzaldehyde. It is generally accepted in the polymer industry that the crude terephthalic acid product should not contain more than about 0.2% of this aldehyde impurity. Reducing the concentration of the 4-carboxybenzaldehyde impurity in the crude product is a difficult proposition since the aldehyde impurity and the terephthalic acid have similar molecular properties. Many purification schemes have been proposed.
In one purification procedure, the terephthalic acid is purified by forming a water-soluble alkali salt of the acid, e.g., diammonium or disodium terephthalate, and then contacting the salt with an aqueous solution containing activated carbon. The activated carbon acts to remove the incomplete oxidation products and metals from the terephthalic acid crude product. The terephthalic acid is regenerated by neutralization.
In another scheme, the crude terephthalic acid product is purified by first vaporizing the crude product in a carrier gas, such as superheated steam. After such vaporization is accomplished, the superheated steam-vapor mix is intimately contacted with a solid particulate terephthalic acid that is at a temperature sufficiently lower than the vaporous mixture and in an amount sufficient to condense out from the vapor the terephthalic acid. After such condensation, there are two phases present, i.e., a solid phase, which comprises the condensed terephthalic acid and the solid particulate terephthalic acid used to effect such condensation, and a vapor phase which contains the more volatile impurities and the carrier gas. A simple separation of the two phases is then performed. An example of the foregoing is shown in U.S. Pat. No. 3,330,863.
Another scheme utilizes steam sublimation of the crude terephthalic acid product and is disclosed in U.S. Pat. No. 3,526,658. This purification scheme includes the continual feeding of finely divided solid particles of the crude terephthalic acid product into a gaseous entrainer stream. The product is fed in a non-tacky condition and the entrainer stream is flowing at a velocity sufficient to substantially immediately entrain and transport the particles. The terephthalic acid is vaporized while it is being carried in the entrainer stream. The carrier stream and its associated contents are indirectly heated to provide the before-mentioned required sublimation. After separating any remaining solid material from the carrier stream, the carrier stream is cooled to condense at least a substantial portion of the terephthalic acid vapor contained therein. It is noted that this cooling is such that a major portion of the more volatile materials in the carrier stream are not condensed. The condensed terephthalic acid product is removed from the gaseous carrier stream.
Oxidation of the 4-carboxybenzaldehyde impurity to produce more terephthalic acid is included in the process described in U.S. Pat. No. 3,658,894. In this process, the crude terephthalic acid product is heated to a temperature sufficient to cause sublimation thereof. The resultant vapor is captured in a reaction zone through which is passed a gas containing molecular oxygen. This molecular oxygen effects the before-mentioned oxidation of the 4-carboxybenzaldehyde.
Yet other purification techniques are disclosed in U.S. Pat. No. 3,362,989, the latter patent describing a fractional sublimation procedure.
A frequently used crude terephthalic acid product purification process employs terephthalic acid aging-recrystallization in a liquid environment of aqueous acetic acid at a temperature of about 285.degree. C. and under elevated pressure. The aging-recrystallization occurs after at least a portion of the 4-carboxybenzaldehyde has been reduced to p-toluic acid by contact with a catalyst in a hydrogen atmosphere.
While all of the foregoing purification methods may be effective in removing 4-carboxybenzaldehyde from a crude terephthalic acid product, they all suffer from one or more of the following: (1) high energy requirements; (2) elaborate processing equipment; and (3) the use of additional chemicals.
Therefore, it is an object of this invention to provide a process for reducing the content of 4-carboxybenzaldehyde in a crude terephthalic acid product which process has a low energy requirement, is simple in operation and in equipment used and which does not use additional chemicals to achieve the purification.